Elyse C. McCormick | February 21, 2025

Is it possible for organisms to be more powerful together than they would be alone? We frequently hear stories of nature that highlight the difficulty of survival. Nature documentaries revel in the drama of predation, the intrigue of competition in its various forms, highlighting the “survival of the fittest” mentality. But are there examples in nature of cooperation? As it happens, we do have examples in nature where two organisms work together for their mutual benefit, something biologists typically call a “mutualism.” And with a little digging, we frequently see that tiny actors make big impacts in these partnerships. Join me in the jungle to learn more about how bullhorn acacia (Vachellia spp.) and acacia ants (Pseudomyrmex spp.) get along, and how these little ants do more than just sting!

In much of Central America, bullhorn acacia are common trees in the understory of the forest, frequently popping up in tree falls where light finally reaches the forest floor. But acacias don’t have to fight for light. When any harmful vines or strangling plants start to grow on the acacia, the trees’ live-in ant protectors come to the rescue, driving away all threats and giving the acacia uninterrupted access to sunlight and nutrients. In return, these small trees provide a home and food for acacia ants. How are the ants such good defenders? They use their very powerful jaws to cut away plants, and their very powerful sting to keep insects and other larger animals away from the acacia’s leaves, preventing predation [1]. The ants are also an interesting adaptation to a lack of chemical defenses. Acacia tree species which don’t host ants produce secondary defense compounds that protect the trees from predators without the aid of the ants. Bullhorn acacia do not produce these compounds; instead, they depend on the ant armies living in their thorns to defend them from the mammals, lizards, and insects that can harm or eat their delicate leaves [2,3]. When scientists removed acacia ants from bullhorn acacias, leaving them defenseless, predators completely defoliated the trees [4]. 

Figure 1. Bullhorn acacia tree, with a focus on the thorns and foliar nectaries. Photo by Elyse McCormick

In return, the acacia tree’s thorns house the ants. Unbelievably, simply having the ant live in these thorns benefits the thorn’s growth. In Nicaragua in 1874, naturalist Thomas Belt noted that when the ants hollow out the acacia thorns, it makes the thorn’s exterior stronger. In trees uninhabited by ants, the thorns became withered and dried into reduced prickles[5]. Strong thorns are indispensable to the tree, both for defending itself and for defending its ants. Not only are the ants securely housed, but they are also fed. The acacia has large nectar-producing glands called foliar nectaries, which provide a flow of syrupy liquid sugar that the ants can constantly access [6]. The acacia also gives another food: Beltian bodies, named for our friend Thomas Belt. These Beltian bodies are modified portions of the leaf, and upon close examination look like small golden pears. The ants harvest these Beltian bodies as they ripen, giving them lasting nutrition throughout the year [5]. 

Figure 2. Golden Acacia ants (Pseudomyrmex spp.) on an acacia tree. Photo by Elyse McCormick.

 In addition to guard duty, these mutualistic ants also protect the acacia from plant pathogens [7]. How? Researchers found that the acacias with an ant symbiosis had fewer instances of bacterial disease than acacias without ants. The scientists then looked at the chemicals ants produce, and then put these chemicals on petri dishes with a variety of bacteria. They found these chemicals, variants of methanol, were strong enough to inhibit many common but powerful types of bacteria. The results of this study suggest that the ants, already protecting the acacia from other threats, are also protecting it against dangerous microbes!

Together, the ants and acacias have co-evolved together to cooperate and ensure each other’s survival. The ants benefit nutritionally from their symbiotic partner; without the acacia the ants would lose highly sought-after nutrients. Thomas Belt [5] said, “In the dry season on the plains, the acacias cease to grow. No young leaves are produced… Then want and hunger overtake the ants… many of the thorns are depopulated, and only a few ants live through the season of scarcity.” In the case of the acacia, if the ant was removed, it would become overgrown with vines and ravaged by insects and other animals ready to eat its leaves. These organisms have evolved to meet the other’s needs, to a point where it is impossible to separate the organisms [6]. Their separation only makes each organism weaker.

Through the acacia and its ants, we see a mutualism so extensive that each organism cannot survive without the other. These special adaptations highlight the ecological complexity which can exist between species. This relationship co-evolved to accommodate the needs of both organisms, resulting in a finely tuned system that shapes the lives of these organisms and the organisms around them. The ants are small but mighty; without these little defenders, the acacia would collapse, ultimately altering the whole forest. One thing is clear, there are certainly powerful examples of cooperation in nature, and that’s certainly something we all need a little more of these days.

References

[1] Forsyth, Adrien; Miyata, Ken. Tropical Nature. New York City: Touchstone. (1984).

[2] Janzen, D. H. “Coevolution of mutualism between ants and acacias in Central America.” Evolution 20(3):  249-275. (1966).

[3] Cronin, G.  Between-species and temporal variation in Acacia-ant-herbivore interactions. Biotropica, 135-139 (1998).

[4] Rehr, S. S. Feeny, P. P. & Janzen, D. H. Chemical Defense in Central American Non-Ant-Acacias. Journal of Animal Ecology 42(2); 405–416. (1973).

[5] Belt T. The Naturalist in Nicaragua. London: J.M. Dent & Sons, LTD. 286 (1874)

[6] Allen, Will. Green Phoenix. Oxford: Oxford University Press. 310 (2001).

[7] González-Teuber M. Kaltenpoth M. Boland W. Mutualistic ants as an indirect defence against leaf pathogens. New Phytologist. 202(2): 640-650.(2014).